usbdebug: Fix boards without EARLY_CBMEM_INIT

The main usbdebug file lib/usbdebug.c was removed from romstage
build with commit f8bf5a10 but the chipset-specific parts were not,
leading to unresolved symbol errors for AMD platforms.

Add a silent Kconfig variable USBDEBUG_IN_ROMSTAGE for convenient
use of this feature.

Change-Id: I0cd3fccf2612cf08497aa5c3750c89bf43ff69be
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3983
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>
diff --git a/src/lib/Makefile.inc b/src/lib/Makefile.inc
index 6110b88..b0d9366 100644
--- a/src/lib/Makefile.inc
+++ b/src/lib/Makefile.inc
@@ -49,11 +49,11 @@
 romstage-$(CONFIG_CONSOLE_SERIAL8250MEM) += uart8250mem.c
 romstage-$(CONFIG_CONSOLE_NE2K) += ne2k.c
 romstage-$(CONFIG_SPKMODEM) += spkmodem.c
+romstage-$(CONFIG_USBDEBUG_IN_ROMSTAGE) += usbdebug.c
 
 ifeq ($(CONFIG_EARLY_CBMEM_INIT),y)
 romstage-$(CONFIG_COLLECT_TIMESTAMPS) += timestamp.c
 romstage-$(CONFIG_CONSOLE_CBMEM) += cbmem_console.c
-romstage-$(CONFIG_USBDEBUG) += usbdebug.c
 endif
 
 romstage-y += compute_ip_checksum.c